In the manufacture of semiconductor devices and other products, ion implantation systems are used to impart impurities, known as dopant elements, into semiconductor wafers, display panels, or other workpieces. Conventional ion implantation systems or ion implanters treat a workpiece with an ion beam in order to produce n- or p-type doped regions, or to form passivation layers in the workpiece. When used for doping semiconductors, the ion implantation system injects a selected ion species to produce the desired extrinsic material. For example, implanting ions generated from source materials such as antimony, arsenic, or phosphorus results in n-type extrinsic material wafers. Alternatively, implanting ions generated from materials such as boron, gallium, or indium creates p-type extrinsic material portions in a semiconductor wafer.
Conventional ion implantation systems include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at a surface of the workpiece to implant the workpiece with the dopant element. The energetic ions of the ion beam penetrate the surface of the workpiece so that they are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particulates.
Ion dose and energy are two variables commonly used to define an ion implantation. The ion dose is associated with the concentration of implanted ions for a given semiconductor material. Typically, high current implanters (generally greater than 10 milliamps (mA) ion beam current) are used for high dose implants, while medium current implanters (generally capable up to about 1 mA beam current) are used for lower dose applications. Ion energy is used to control junction depth in semiconductor devices. The energy of the ions which make up the ion beam determine the degree of depth of the implanted ions. High energy processes, such as those used to form retrograde wells in semiconductor devices, typically require implants of up to a few million electron volts (MeV), while shallow junctions may only demand energies below 1 thousand electron volts (keV).
The continuing trend to smaller and smaller semiconductor devices requires implanters with ion sources that serve to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy levels permit shallow implants. Source/drain junctions in complementary metal-oxide-semiconductor (CMOS) devices, for example, require such a high current, low energy application.
A typical ion source for obtaining atoms for ionization from a solid form is comprises a pair of vaporizers and an ionization chamber. Each of the vaporizers is provided with a crucible in which a solid element or compound is placed and which is heated by a heater coil to vaporize the solid source material. Vaporized source material passes through a nozzle, or compressed gas may be fed directly into the ionization chamber, wherein the gaseous/vaporized source material is ionized by an arc chamber filament that is heated to thermionically emit electrons.
Conventional ion sources utilize an ionizable dopant gas which is obtained either directly from a source of a compressed gas or indirectly from a solid which has been vaporized. Typical source elements are boron (B), phosphorous (P), gallium (Ga), indium (In), antimony (Sb), and arsenic (As). Most of these source elements are commonly used in both solid and gaseous form, except boron, which is almost exclusively provided in gaseous form, e.g., as boron trifluoride (BF3).
In the case of implanting boron trifluoride, a plasma is created which includes singly charged boron (B+) ions. Creating and implanting a sufficiently high dose of boron into a substrate is usually not problematic if the energy level of the beam is not a factor. In low energy applications, however, the beam of boron ions will suffer from a condition known as “beam blow-up”, which refers to the tendency for like-charged ions within the ion beam to mutually repel each other. Such mutual repulsion causes the ion beam to expand in diameter during transport, resulting in vignetting of the beam by multiple apertures in the beamline. This severely reduces beam transmission as beam energy is reduced.
Decaborane (B10H14) is a compound which is an excellent source of feed material for boron implants because each decaborane molecule (B10H14), when vaporized and ionized, can provide a molecular ion comprised of ten boron atoms. Such a source is especially suitable for high dose/low energy implant processes used to create shallow junctions, because a molecular decaborane ion beam can implant ten times the boron dose per unit of current as can a monatomic boron ion beam. In addition, because the decaborane molecule breaks up into individual boron atoms of roughly one-tenth the original beam energy at the workpiece surface, the beam can be transported at ten times the energy of a dose-equivalent monatomic boron ion beam. This feature enables the molecular ion beam to avoid the transmission losses that are typically brought about by low energy ion beam transport.
An exemplary ion implantation system 10 is illustrated in FIG. 1, wherein the ion implantation system comprises a terminal 12, a beamline assembly 14, and an end station 16. The terminal 12 includes a suitable ion source 20 powered by a power supply 22, wherein terminal is configured to produce and directs a molecular decaborane ion beam 24 through the beamline assembly 14, and ultimately, to the end station 16. The beamline assembly 14, for example, has a beamguide 26 and a mass analyzer 28 associated therewith, wherein a dipole magnetic field is established to pass only ions of appropriate charge-to-mass ratio through an aperture 30 at an exit end of the beamguide 26 to a workpiece 32 (e.g., a semiconductor wafer, display panel, etc.) disposed in the end station 16.
During the molecular decaborane ion implantation into the workpiece 32, however, various contaminants (not shown) are typically generated from the molecular decaborane ion beam 24 over time, and strike and adhere or deposit on various components 34, such as the aperture 30 and Faradays 36 disposed along the beam path. Collisions of ions with the various components 34, for example, can further sputter contaminants (not shown) onto other surfaces 38 situated along the beam path. Implementations of decaborane ion sources, however, have led to unique particle contamination issues, as dissociation of the decaborane molecule and fragmentation of the B10HX+ desired parent ion can occur, and substantially large particles can quickly accumulate on various components 34 and surfaces 38 within the ion implantation system 10.
Conventionally, contaminants are removed from the ion implantation system by a manual cleaning of the components 34 and surfaces 38, wherein the various components are removed, cleaned, and then replaced. Such cleaning is typically performed by an operator during scheduled maintenance of the ion implantation system. Manual cleaning is typically costly, not only in terms of time and labor attributed to the operator, but also in terms of decreased efficiency and yield of the ion implantation system 10 due to increased down-time associated with the maintenance.
As an alternative, etchant gases, such as a highly reactive halide or fluorine gas, have been introduced into the ion implantation system 10 in an attempt to remove the contamination by chemical reaction between the contaminants and the etchant gases. This solution, however, typically requires a change of gases in the ion implanter 10, wherein the source material gas used for implanting ions into the workpiece 30 is purged from the ion implanter, the etchant gas is then used to remove the contamination, and then the etchant gas is further purged from the implanter prior to introducing source material gas again for processing of another workpiece. Such an etchant gas may remove some or all of the contaminants, however, the use of the etchant gas typically necessitates a considerable amount of time to not only introduce the gas, but to allow time for the etchant gas to react with and etch the contaminants, as well as time spent removing the etchant gas from the ion implantation system once etching is complete. The use of such etchant gases may thus decrease the efficiency of the ion implantation system 10, thereby decreasing a throughput of the implanter.
Accordingly, it is an object of the present invention to provide a method and apparatus to sufficiently reduce particle contamination in a molecular ion beamline assembly in a time-efficient manner, wherein efficient contaminant mitigation can be facilitated, and wherein high throughput and highly reliable molecular ion implantation into a workpiece can be achieved.